The conical geometry is inherently stiff as axisymmetric external forces applied to it manifest themselves as tensional stresses in the material. Advantageously, this permits the successful use of very thin membrane material.
In a competitive marketplace, there is an ever-increasing requirement to obtain improving performance from cone loudspeakers. FIG. 1(a) shows views of the cone of a cone loudspeaker, and FIG. 1(b) shows its pressure response when neck driven in the conventional manner with a 93 mm diameter mouth end radiating into a 2 pi steradians infinite acoustical region. The pressure is plotted at 46 positions at Im from the loudspeaker and at 2 degree angular increments. It may be seen from FIG. 1(b) that above approximately 1.5 kHz the pressure response becomes irregular and resonances appear as the cone is driven beyond the limits of its rigidity and exhibits non-rigid behaviour. Non-rigid behaviour is undesirable at it results in non-uniformity in both the pressure and direction response of the loudspeaker.
It has been long known that the bandwidth of rigidity in a loudspeaker diaphragm may be extended by driving the diaphragm at the node of the first mode of vibration (“nodal driving”). Nodal driving was disclosed in JP57068993 which shows a flat plate diaphragm being driven at the node of the first mode of vibration which, for a circular diaphragm, is a circle around the diaphragm. This approach, although long known, has not, however, been applied to a cone loudspeaker. The geometry of the cone naturally places the node of its first mode of vibration towards its mouth end which would necessitate the use of a large voice coil. The use of a large diameter coil has a negative impact on efficiency and increases the costs of the associated magnet system and coil assembly, which has considerably limited the practicality of nodal driving. The universal practice in the art has hitherto been to drive the cones from their neck.
GB308,318 discloses a loudspeaker with a frusto-conical diaphragm, driven at both the neck of the diaphragm and also at a concentrically-spaced location outside the diaphragm. The intention is to send high-frequency signals to the inner (neck) drive and low-frequency signals to the outer drive which is then located at a node for the high-frequency signal. Thus, it does not in fact suggest nodal driving, as neither drive is located at a node of the diaphragm for the signal that the drive in question is supplying. Nor, indeed, is the diaphragm driven at a node of the first mode of vibration; the node at which it is driven is a node of a higher mode corresponding to the higher frequency drive. Further, no reinforcement of the diaphragm is disclosed, and the outer drive is therefore spaced at a considerable diameter, giving rise to the problems noted above.
U.S. Pat. No. 5,323,469 discloses a loudspeaker with a conical diaphragm driven by a rearwardly extending voice coil former attached to a throat portion of the diaphragm. Additional stabilisation is provided for the diaphragm in the form of a second cone extending radially outwardly from the former, behind the diaphragm, and attaching to the diaphragm at the first nodal point. The interface region by which the voice coil former drives the diaphragm therefore extends between and includes both the throat and the nodal point, and the diaphragm is not therefore nodally driven. The additional stabilisation also extends the depth of the loudspeaker unnecessarily, provides little significant stiffening of the diaphragm away from the nodal point, and is not apt to allow tailoring of the stiffness characteristics of the diaphragm.